Cosmic microwave background experiments from space: why, how, and when

We review the importance of Cosmic Microwave Anisotropy studies in Cosmology, and the impact of space-based observations in this field. Balloon-borne experiments have recently provided important breakthroughs, like the measurement of the density parameter and the evidence for acoustic oscillations in the primeval plasma. Forthcoming space missions will provide high precision observations, allowing the measurement of the cosmological parameters and of the composition of the Universe